You'll probably have to connect 2 47uF electrolytic capacitors in parallel to get 94uF capacitance.

Also, as soon as you detect that the power has been cut off, first thing you should do is set all the pins as inputs. That will cut off any leaking power. From there one second should be enough to save everything. Once that works, you can find out how much exactly it takes to cut off all the pins, and save the data, and from there you should be able to recalculate needed capacitor value to provide you with enough power. But remember, capacitor's charge should still be well above the brown-out voltage, otherwise uC will turn off.